Computer Networks
Tutorial #18
- What are the advantages of a public key cryptosystem compared
to a single key system? Why is the problem of key management
almost as difficult in public key systems as it is in single key
systems?
- In the lecture, a demonstration was given of a RSA-like
public key encryption system, albeit one using very small primes.
The numerical results were given without justification. Verify
the correctness of the calculations presented there.
- In the RSA example given in the lecture, what aspect of the
system makes it difficult to discover
KS
(the
decryption, or private key) given that you know
KP
, the public encryption key?
- In the public-key authentication protocol given in the
lecture notes,
in message 3 (sent from A to B),
RB
is encrypted with KS
. Is this
encryption necessary, or would it have been adequate to send it
back in plaintext?
- What is the difference between a digital signature and
a message digest? What are the advantages and disadvantages
of each?
- In the
lecture notes,
it was claimed that no one can generate two messages that have the
same message digest. How can the system designer ensure this?
- The Unix system uses a scheme with some similarities to a
message digest for storage of user passwords. In what ways are these
similar? NB: if you don't use Unix, or use it infrequently, you may be
excused from this question.
- (Advanced - maths majors only. This will not be on the
exam!) Using the RSA cryptosystem with
p = 7
and q = 11
, list 3
legal values for D
.
- (Advanced - maths majors only. This will not be on the
exam!) Again using an RSA cryptosystem, this time with
p = 13, q = 31 and D = 7
,
find E
.
Practical Exercises:
- Much earlier in the semester we discussed
telnet
,
the generic protocol for remote login. We also mentioned
rlogin
. However, both of these have effectively been
replaced by a far superior software utility, ssh
, the
"secure shell". You must stop using telnet
,
rlogin
,rsh
or whatever you are currently
using to do remote login and use ssh
instead.
Seriously!
- Find out about and use PGP. Everyone should do this, even if you're
not intending to answer the PGP question on assignment 3.
- Investigate MD5.
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Phil
Scott